1. The Field of the Invention
The present invention relates generally to x-ray tubes. More specifically, embodiments of the present invention relate to an improved x-ray tube window and frame that exhibit improved mechanical and x-ray transmission characteristics.
2. The Relevant Technology
X-ray tubes typically utilize an x-ray transmissive window formed in the housing of the x-ray tube that permits x-rays produced within the tube to be emitted. The window is typically set within a window frame, and is located in the side or in the end of the x-ray tube. The window is typically a disk-shaped plate comprised of beryllium or similar materials that are x-ray transmissive.
FIG. 1A is a simplified cross-sectional illustration of an x-ray tube 2 as used in the prior art having a window 4 positioned in an end of the x-ray tube. An x-ray tube housing 3 defines an evacuated enclosure 5 that encloses an electron source or cathode 13, and an anode 14. As is shown, the window 4 is affixed within the wall of the housing with a window frame 8, which is illustrated as being structurally integrated within the x-ray tube housing. The window 4 separates the vacuum of evacuated enclosure 5 of the x-ray tube from the normal atmospheric pressure found outside the tube, and yet enables x-rays generated from the anode 14 to exit the x-ray tube 2 and strike an intended target 16.
Although window thickness will vary depending on the particular x-ray tube application, windows are typically very thin, often measuring 0.08 millimeters or less. In particular, a window with a reduced thickness is generally desired so as to minimize the amount of x-rays that are absorbed by the window material during x-ray tube operation.
While a thinner window is desirable, a thin window affixed to a window frame that is integrally mounted to an x-ray tube housing is typically subjected to deforming stresses created as a result of the large pressure differential that exists on either side of the window. Such deforming stresses are non-uniformly distributed over the surface of the window and can produce cracking in the window and leaks between the window and the window frame. This can cause the x-ray tube housing to lose its vacuum, and render the x-ray device inoperable.
For example, FIG. 1B depicts a typical side mounted window-type x-ray tube 10 in cross section, having a window 12, cathode 13, anode 14, and intended target 16. The window 12 is depicted in phantom to illustrate its typical deformed state due to the pressure differential existing across its inner and outer surfaces. Again, this can result in cracking and consequent loss of vacuum.
FIG. 2A depicts a close-up cross-sectional view of a typical prior art window frame 20 of an x-ray tube 18, with a window 22 disposed therein. The window 22 is typically brazed or diffusion bonded to a support flange 21 of the window frame 20. The support flange 21 typically extends parallel to the plane in which window frame 20 is situated. The deflection of window 22 inward toward evacuated enclosure 26 (defined by x-ray tube housing 19) is caused, as mentioned earlier, by the pressure differential existing across the inner and outer surfaces of the window, which surfaces separate the vacuum tube interior from the normal atmospheric pressure exterior designated at 24. The deflection of the window 22 is especially apparent at the junction 28 of the support flange and the window,. and is the area that is subjected to a relatively higher level of mechanical stress.
FIG. 2B illustrates a view of the window 22 supported by window frame 20 from the perspective of lines Axe2x80x94A in FIG. 2A. The view further illustrates the area of junction 28 and the area of transition from where window 22 is supported by support flange 21, to the area where the window is not supported.
The deflection of window 22 worsens after x-ray tube 18 is processed in high temperature environments during tube manufacture. One example of such high temperature processing is air baking. This process involves heating the x-ray tube (with the window and windrow frame attached to the tube housing) to approximately 450 to 475 degrees Celsius for a given amount of time. This imposes a fair level of thermal stresses in the window area and, when combined with the stresses caused by the pressure gradient existing over the surfaces of window 22, further result in the surface of window 22 being. stressed and deformed inwardly toward evacuated enclosure 26. Again this can result in cracking of the window and air leaks into tube housing 18, and limit the operational life of the x-ray device.
One approach used to overcome this problem is. to increase the thickness of the window. Thicker windows are inherently stronger and less susceptible to stress and the resultant cracks. However, a thicker window is less transmissive to. x-rays, especially those of lower energy. This can be especially problematic in low power x-ray tubes.
Consequently, it would be an improvement over the state of the art to provide an x-ray tube window that suffers less from the effects of deforming stress, and yet maintain sufficient transmissivity to x-rays. Also, it would be desirable to provide a window frame that decreases the incidence of stress on the window mounted therein, thereby increasing the useful life of the x-ray tube. Further it would be an improvement to provide a window frame and window that can be assembled and manufactured in a manner that reduces the incidence of mechanical and thermal damage that traditionally occurs during the manufacturing process.
Given the problems present in currently available x-ray tubes, it is a primary objective to provide an x-ray tube that is more resistant to mechanical stresses present in an operating x-ray tube.
A related objective is to minimize the occurrence of cracks in the window and thereby prevent vacuum leaks from occurring.
Also, it is an objective to provide a method and apparatus that minimizes the degree of deforming stresses that are imposed on an x-ray tube window during certain manufacturing steps.
These and other objectives, features and advantages are provided in embodiments of the present invention, which are generally directed to a new x-ray tube window and window frame. In particular, the improved frame and window reduce bending/deflection stresses on the window.
In one presently preferred embodiment, a window frame is provided that utilizes a support flange for supporting the x-ray tube window. Preferably, the support flange is angled towards the evacuated enclosure cavity of the x-ray tube. In this way, the deflection to which the window is typically subjected is anticipated and compensated for, thereby reducing or eliminating deflection stresses that are otherwise imposed on the window as a result of pressure differentials and thermal stresses.
In one presently preferred embodiment, the support flange is oriented at an angle that is consistent with the degree of deflection that the window would otherwise experience in a deflected state. The angle of the window frame support flange is constructed such that at a given radial distance from the center of a window, the primary stress component acting on the window is tension, thus minimizing torsion or other bending stresses acting thereon.
This reduction in bending stress along the interface with the support flange minimizes the occurrence of cracks in the window, or along the attachment interface with the flange. This minimizes vacuum leakage, and results in an extended tube lifetime. Also, a reduction in stresses on the window make it possible for a thinner window to be utilized, thus enabling a higher level of x-ray transmission through the window. Also, an increased level of x-rays can be emitted from the x-ray tube without increasing the power level of the x-ray device.
These and other objects and features of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.